This PR adds focused unit tests for aggregate_by_field in OceanBase
memory utilities to improve behavior coverage for real-world input
shapes.
- Adds test coverage for list-valued aggregation fields, including
whitespace trimming and skipping invalid list entries.
- Adds test coverage for scalar field values to ensure blank/non-string
values are ignored.
- Confirms aggregation output remains correct and stable for
mixed-quality message payloads.
### Why this helps
It strengthens regression protection for aggregation logic used by
memory retrieval flows, with no production code changes and minimal
review risk.
### What problem does this PR solve?
- Add aggregation_utils.aggregate_by_field for pure aggregation logic
- Wire OBConnection.get_aggregation to use it (unwrap tuple, pass
messages)
- Add unit tests for aggregate_by_field (no DB/heavy deps)
### Type of change
- [x] New Feature (non-breaking change which adds functionality)
